This project is to compile a new database of hourly night-time parameters characterizing the mesosphere and lower thermosphere (MLT) based on reanalysis of historical data acquired by the Colorado State University sodium lidar system. Temperature measurements obtained between 1990 and 2001will be re-processed to the same spatial resolution (2 km) as more recent data, thus enabling longer-term investigations than previously possible, while data acquired from 2002 to 2009 will be re-processed with a spatial resolution of 1 km and a temporal resolution of 5-15 minutes for gravity wave studies. Additional project goals are to conduct scientific investigations of gravity wave dynamics using the high-temporal resolution data and to use the long-term temperature data set to investigate mesospheric responses to variations in solar activity and the El Nino Southern Oscillation. The re-analyzed data will be made available to the CEDAR Database for further community use.
How do you measure temperatures and wind speeds in the atmosphere at altitudes ten times higher than passenger jet airliners fly? Why with sodium lidars! Sodium lidars depend upon a naturally occurring layer of atomic sodium (with sodium atoms separated by about 0.001 meter and the atoms originate in the atomization of meteors which enter the atmosphere continuously) in the atmosphere (at altitudes of about 80 to 105 km over the entire earth) which fluoresces when absorbing photons from a laser. The atoms resonantly absorb a photon of one color and then, a very short time later, will emit a photon of a different color. The sodium lidar detects some of those photons which happen to return to the earth. Taking advantage of the resonance phenomena, the cross-sections (probabilities) for interacting with sodium atoms are about 15 orders of magnitude (a million billion times) larger than the cross-sections for interacting with other gases, e.g. nitrogen. The variability of the velocities of the sodium atoms is a measure of the temperature of the atoms and shows up as a Doppler broadening of frequencies (spread of colors) of the light emitted by the atoms. A general drift of all the atoms with the wind results in a Doppler shift in the emitted frequencies (shift in the color) which is proportional to the wind speed. The system requires a very special laser that emits photons with a very narrow band width (i.e. range of frequencies or colors) in a pulse. Detecting the time for photons to return determines where in the atmosphere the interaction with the sodium took place and allows properties to be measured as a function of altitude. Of the approximately one billion, trillion (1e17) photons emitted in laser pulse lasting about ten billionths of a second (ten nanoseconds), the system telescopes detect just a few hundred photons that are bringing back information about the speeds and temperatures of the sodium atoms in the atmosphere. Narrowband sodium lidars for measuring mesopause region temperatures were pioneered by Fricke and von Zahn in 1985 with a pulsed dye laser system and later by She and co-workers in 1990 with a hybrid cw-pulsed dye laser system at Colorado State University (CSU) operating at two frequencies. The latter was upgraded to a system capable of measuring both temperature and wind in 1994 by the addition of a third frequency, and of observation under sunlit conditions in 1996 by the addition of a Faraday filter. The CSU system Na lidar system performed full-diurnal-cycle (24 hours continuous) observations beginning in 2002. The sodium lidar system at CSU at Fort Collins, CO (41N, 105W) was used to measure temperatures, sodium densities, and, in some cases, one or two components of wind velocities at altitudes from 80 to 105 km (mesopause region) during 1000 nights and some days from 1990 to 2010. This project reanalyzed this data using a uniform space-time resolution of 2 km and 1 hour. This dataset will be especially useful for long-term climatic changes and is available at http://madrigal.physics.colostate.edu/htdocs or http://madrigal.haystack.mit.edu/madrigal. Capabilities were developed to analyze some of the best data at spatial resolutions of 1 km and temporal resolutions of 10 minutes and 2.5 minutes. This data set will be of special significance for studies of gravity waves and tidal interactions and is still under development.